Telephone exchanges with speech storage equipment are already known and are mostly called voice-mail equipment in the profession. Such voice mail devices are very explicitly described by EP 0,087,849 and EP 0,336,524, for example. A method and a device for storing telephone messages in a private telephone branch exchange are also known from EP 0,304,653.
The task of these devices is for called subscribers, who cannot be reached at the moment, to initiate a fixed call transfer to the voice mail device, or for the device to initiate a call transfer after a predetermined waiting time, if the called subscriber does not answer the call.
If a call is transferred to the voice mail device and it answers the call, the telephone set signals the number of the called subscriber by means of Dual Tone Multiple Frequency (DTMF) signals, which automatically connect the calling subscriber with the personal mailbox of the called subscriber and allows him to record his voice message.
Exchange installations with call transfer have been known for a long time. All the devices in these installations are reset to their starting condition after the voice connection is terminated. As mentioned, if the called subscriber has not activated a call transfer to the voice mail device, the call transfer of a new call is only activated after a predetermined time has passed.
The known devices have the disadvantage that if the subscriber is not ready to answer, each new call first reaches the subscriber station and is only connected with the voice mail device if the receiver of the called subscriber's telephone set has not been lifted or answered within the predetermined waiting time. This is considered to be disturbing to other persons who are present in the room at that time. In addition, the time span before the call is connected passes without being used by the calling subscriber, i.e., in a wasteful manner.